A monarch butterfly, which has seen a steep decline in population numbers, lands on a blooming wildflower.

A monarch butterfly, which has seen a steep decline in population numbers, lands on a blooming wildflower.

Photo: Tom Stienstra, Brian Murphy / Special To The Chronicle

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Pacific Grove is home to the Monarch Grove Sanctuary. Each winter, thousands of Monarch Butterflies cluster together on the pines and eucalyptus of the Sanctuary. Arriving in October, these hardy insects will over winter until February when they they mate just before St. Valentines day, then they will join the spring Monarch migration, spreading northward and eastward, hunting for milkweed plants on which to lay their eggs. less

Pacific Grove is home to the Monarch Grove Sanctuary. Each winter, thousands of Monarch Butterflies cluster together on the pines and eucalyptus of the Sanctuary. Arriving in October, these hardy insects will ... more

Photo: Frederic Larson, The Chronicle

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A struggling Monarch butterfly is picked from the ground beneath a eucalyptus tree where it had fallen. At the Ardenwood Historic Farm in Fremont, Calif. hundreds of mysterious wintering Monarch butterflies are coping with a cold winter Sunday January 6, 2013. less

A struggling Monarch butterfly is picked from the ground beneath a eucalyptus tree where it had fallen. At the Ardenwood Historic Farm in Fremont, Calif. hundreds of mysterious wintering Monarch butterflies are ... more

Western monarch butterflies, which crowd trees along the California coast every winter and flush them with color, have declined so dramatically since the 1980s that the species will likely go extinct in the next few decades if nothing is done, scientists said Thursday in a population study of the treasured creatures.

Fewer than 300,000 of the brilliant orange and black insects were counted last year at some 300 locations stretching from Marin County to the Baja California peninsula, where millions of wintering monarchs historically took up shop, according to the report published in the journal Biological Conservation.

“We believe there were at least 10 million butterflies in many of the years during the 1980s,” said Cheryl Schultz, an associate professor of biological sciences at Washington State University and the lead author of the study. “It’s gone down from 10 million to 300,000. That’s why we were so shocked. We did not expect it to be that sharp of a decline.”

The U.S. Fish and Wildlife Service, alarmed by an estimated 75 percent drop in the population just since the early 2000s, funded the latest study to help officials decide whether to list the monarch under the Endangered Species Act.

The decline is similar to that seen among the more abundant eastern monarchs, which spend their winters in Mexico before heading back across the United States and settling as far north as Canada. That population is famous because the butterflies form a blanket over trees, turning whole sections of forest into a kaleidoscope of color, the insects so abundant that human beings can hear the sound of their flapping wings.

Eastern monarchs have declined more than 90 percent since 1996, when scientists estimated there were 1 billion nesting in the trees. Last winter, 78 million eastern monarchs were counted in Mexico, compared with 100 million the year before.

The study published Thursday represents the most comprehensive measurement of monarch population declines in the west. Earlier counts only went back to 1997, so Schultz and her colleagues used historic observations and developed a statistical method to estimate the population back to 1981.

What they found is that the California population, first observed by a Russian expedition looking for a passage across the Arctic Ocean in 1816, is declining at an average of 7 percent a year, slightly more than the 6 percent drop seen in the eastern monarch population, said Schultz and her co-authors at the nonprofit Xerces Society, the University of Georgia and Tufts University in Massachusetts.

The monarch, one of the largest butterflies in the world, is found throughout North America. Over the years, it has expanded its range around the globe, including to Hawaii, New Zealand and Australia. It is extremely susceptible to changes in habitat and weather and to toxins in the environment.

The highest concentrations of western monarchs arrive in the Monterey and Pacific Grove areas in November and spend the winter living in clusters on pine, cypress and eucalyptus trees. They leave in February and March, spanning out to breed in Northern California, Nevada, Oregon, Idaho, Washington, Arizona and Utah.

Recent studies have blamed the decline of the eastern population on urban sprawl and a lack of milkweed and other nectar-bearing flowers along the migratory route through the Midwest.

Nobody knows exactly what is killing off the western monarchs, but a lack of milkweed is clearly one of the problems, according to researchers. Schultz said the western population is also losing habitat to development and is suffering due to pesticides, herbicides and changes in the climate.

“One of the pieces we’ll be working on is to get a sense of what’s really driving these declines,” she said.

The winter migration of the monarch is one of the most remarkable of any species. It is not clear how much the two groups of butterflies mix during breeding season, but scientists believe the eastern and western migratory populations divide at the Rocky Mountains when they head south for the winter.

The western group spends much of the winter in California while the eastern population winters in Mexico, more than 2,500 miles from where it started.

By winter’s end, the males have died and the females head back north. But each butterfly lasts only a couple hundred miles, before laying eggs on milkweed as a final act before death. The trip back is essentially a relay race involving generations of adult butterflies, which feed on flowers along the way before breeding and dying.

Scientists still can’t figure out how the butterflies pull off this seemingly magical bit of genetic imprinting.

“It’s alarming and shocking to see declines this significant, but I think there is time to turn the trend around,” said Schultz, who pointed to the story of Oregon’s Fender’s blue butterfly. The species fell to 1,500 individuals in the 1990s before rebounding, and today is back up to 28,000 insects after 20 years of work by a public, private and nonprofit partnership.

“It will take a commitment, hard work and it will take time, but we need to start taking action, and we need to do it now,” she said. “There is so much energy and interest in the monarch. It’s an iconic butterfly that everybody knows. There is really good potential to make this happen.”

Western monarch butterflies spend the winter in more than 300 forested groves along the California coast, including large populations in Riverside and Los Angeles counties, Pacific Grove, Monterey and at Natural Bridges State Beach in Santa Cruz.

They can normally be seen from November to March.

The California winter population declined an estimated 90 percent between 1997 and 2009. There are now roughly 300,000 monarchs in California compared with an estimated 10 million in 1981.